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Blood Pressure Regulation

Faisal I. Mohammed, MD,PhD

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Objectives

Outline the short term and long term regulators of BP

Know how baroreceptors and chemoreceptors work

Know function of the atrial reflex.

Know function of CNS ischemic reflex.

Know the role of Epinephrine, Antidiuretic hormone (ADH), Renin-Angiotensin-Aldosterone and Atrial Natriuretic Peptide (ANP) in BP regulation

Know the role of Kidney-body fluid system in long term regulation of BP

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Shygmomanometry

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Measurement of Systolic and

Diastolic Pressures

Auscultatory method is the most commonly used method for measuring systolic and diastolic pressures.

When cuff pressure reaches systolic pressure, one begins to hear tapping sounds in the antecubital artery; as the cuff pressure reaches diastolic pressure, one hears muffled sounds and then Korotkoff sounds disappear.

Mean arterial pressure can be estimated by adding 40% of systolic pressure to 60% of diastolic pressure.

Cuff Pressure < 80

Cuff Pressure > 120

NO FLOW

FREE FLOW

Effect of Cuff Pressure on Brachial Blood Flow

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Measurement of Blood Pressure

Use of Korotkoff Sounds

120 100 80

150

100

50

0

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Mean Arterial Pressure

Mean Arterial Pressure (MAP)

0.0 0.3 0.8

)/(.1

2

12 t

tttdtdpMAP

t1 t2

MAP= 1/3 systolic pressure

+ 2/3 Diastolic pressure

MAP= Diastolic Pressure

+1/3 Pulse Pressure

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Blood Pressure Regulation

Mean Arterial Pressure (MAP) = 1/3 systolic

pressure + 2/3 diastolic pressure

MAP= Diastolic Pressure+1/3 Pulse Pressure

TPR

MAPCO

TPRCOMAP

Arterial Pressure = Cardiac Output xTotal Peripheral Resistance

Arterial Pressure can be increased by:

• Constricting almost all arterioles of

the body which increases total

peripheral resistance.

• Constricting large vessels of the

circulation thereby increasing

venous return and cardiac output.

• Directly increasing cardiac output

by increasing heart rate and

contractility.

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Autonomic Nervous System

Sympathetic nervous system

is important in control of circulation.

Parasympathetic nervous

system is important in

regulating heart function.

Sympathetic Innervation of Blood Vessels

Sympathetic nerve fibers

innervate all vessels except

capillaries and precapillary

sphincters and some metarterioles.

Innervation of small arteries and

arterioles allow sympathetic nerves

to increase vascular resistance.

Large veins and the heart are also

sympathetically innervated.

Parasympathetic nervous system is

mainly important in control of heart

rate via the vagus nerve.

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Short term regulation of BP 1. Baroreceptors or pressoreceptos (High pressure)

2. Carotid and Aortic Baroreceptors

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Short term regulation of BP cont…

1. Baroreceptors or pressoreceptos (High pressure)

The Vasomotor Center (VMC)

The VMC transmits impulses

downward through the cord to

almost all blood vessels.

VMC is located bilaterally in the

reticular substance of the medulla

and the lower third of the pons.

The VMC is composed of a

vasoconstrictor area, vasodilator

area, and sensory area.

Arterial Baroreceptor Reflex

Important in short term regulation of arterial pressure.

Reflex is initiated by stretch receptors called baroreceptors or pressoreceptors located in the walls of the large systemic arteries.

A rise in pressure stretches baroreceptors and causes them to transmit signals to the VMC and feedback signals are sent via the automonic nervous system to the circulation to reduce AP back to normal.

Blood

Pressure

Arterial

Baroreceptors

Vasomotor

Center

Response of the Baroreceptors to Arterial Pressure

Carotid sinus baroreceptors respond to

pressures between 60 and 180 mmHg.

Baroreceptors respond to changes in

arterial pressure.

Baroreceptors reflex is most sensitive

at a pressure of 100mmHg.

As pressure increases the number of

impulses from carotid sinus increases

which results in:

1) inhibition of the vasoconstrictor

2) activation of the vagal center

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Baroreceptor mechanism

Functions of the Baroreceptors

Maintains relatively constant pressure despite

changes in body posture.

Decrease

Cardiac Output

Sensed By

Baroreceptors

Supine Standing

Decrease

Central

Blood Volume

Vasomotor

Center

Sympathetic

Nervous Activity

Decrease

Arterial Pressure

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Functions of the Baroreceptors

• Opposes either increases or decreases in arterial pressure thereby

reducing daily variations in arterial pressure.

• They are unimportant in long term control of arterial pressure

because the baroreceptors adapt.

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Effect of Parasympathetic and Sympathetic Nervous Systems

on Factors that Influence the Mean Arterial Pressure

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Short term regulators cont…

2. Low Pressure Baroreceptors:

• Located at Rt. Atrium, Rt. Ventricle ? Pulmonary artery

• Sensitive to changes in Volume

• An increase in volume increase in venous return increase in cardiac output increase in MAP and vice versa

• This work through atrial-hypothalamic reflex through stimulating Anti Diuretic Hormone (ADH) –Vasopressin- in case of low BP or inhibiting ADH in case of high BP i.e this will affect the volume and so the BP through its effect on urine output and also on TPR since ADH (Vasopressin) is strong vasoconstrictor.

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Short term regulators cont…

Low pressure baroreceptors work also

through affecting the Glomerular Filtration

Rate (GFR) by causing Afferent arteriolar

dilation in case of high BP due to high

volume or Vasoconstriction in case of low

BP due to low volume

Affecting the GFR means affecting the

urine volume

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Negative Feedback Regulation of Baroreceptor Reflex

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Short term regulation of BP cont…

3. Chemoreceptors

Carotid and Aortic Chemoreceptors

Chemoreceptors are chemosensitive cells sensitive to oxygen lack, CO2 excess, or H ion excess.

Chemoreceptors are located in carotid

bodies near the carotid bifurcation and on

the arch of the aorta.

Activation of chemosensitive receptors

results in excitation of the vasomotor center.

Chemoreceptors are not stimulated until

pressure falls below 80mmHg.

Sympathetic

activity Chemoreceptors VMC

O2

CO2

pH BP

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Thank You